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Abstract

Part of the Archives for the History of Quantum Physics collection, which includes tapes and transcripts of oral history interviews conducted with circa 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Niels Henrik David Bohr, Max Born, Léon Brillouin, Louis de Broglie, Hendrik Brugt Gerhard Casimir, Paul Drude, Paul Ehrenfest, Albert Einstein, Walter Friedrich, Dirk ter Haar, Fritz Hasenöhrl, Werner Heisenberg, David Hilbert, Heinz Hopf, Herman Julius, Felix Klein, A. Kleiner, Paul Langevin, Max Theodor Felix von Laue, Fritz London, Gustav Mie, Walther Nernst, A. A. Noyes, Max Planck, W.C. Röntgen, Paul Scherrer, Erwin Schrödinger, Arnold Sommerfeld, Pierre Weiss, Max Wien, Wilhelm Wien, Zangger, Zermelo; Eidgenössische Technische Hochschule Zurich, Massachusetts Institute of Technology, Universität Leipzig, and Universität München.

Transcript:

Debye:

What I always say is that at the time in Aachen I had time. Although I had to make some drawings, I had time to go into the library. I remember I read in the library about that time about (???), and such things. I read the Comptes Rendus and these things. They had nothing to do with the teaching, and I had time to do that. And if you take students here in the United States, they never have tine.

Uhlenbeck:

They have to follow the courses.

Debye:

Well yes, but the courses should not be occupying the whole time. Now let’s see what else is on the outline. You wanted to know about Munich, what else? Oh yes, “That was the role of the first Solvay Congress in focusing attention’ and so on? I was not there, so I don’t know. “The state of the X-ray problem with Laue .“ I told you about that....

Kuhn:

Was there any significant difference between the sort of curriculum one had at Zurich and the sort you had at Munich?

Debye:

No, because I made it. You see I did not have to do anything. The only thing which was said to me was that I should also give a course in relativity. But otherwise it was the regular scheme... It was the regular scheme which I introduced, and nobody said anything against it. The only difference was that you see now, since I stood well with Kleiner, I could have my part in the laboratory. And that was the nice thing about it. There were also some very bad students there, some which came from Poland and so on. These were not very good. We had trouble with them.

Uhlenbeck:

Who were your early students?

Debye:

Now there was one. I tried this morning to remember his name. I cannot remember his name. He was a Hungarian. That was the time when I was talking about dipoles. My main point was that dipoles had also to show the dispersion, which had been found by Drude already. You know, dispersion in long waves. And he helped with that. And he was later a professor in Budapest, but I forget his name. It might have been something like Ortvay.

Uhlenbeck:

But you had no students who did theoretical work with you?

Debye:

No, no. You see I had one student who was later in London with Bragg. Miller was his name, Mül1er, really. And he was the helper, the main man was the older, the father of Bragg, in Ablemarle Street. And he made these well he was more experimental, and he made these X-ray proofs with the rotating anti-cathode, he could put much more energy in it. That was my first doctors’ candidate in Zurich• He lived in England later on.... He died already. This Ortvay, if his name is Ortvay, was the man who helped me with this dispersion effect. That was the clinching point for me, the dispersion. That you could get an anomalous dispersion. Well, that was what Drude had found. That there is an anomalous dispersion. In water, the dielectric constant going from 80 to 3 in a range of frequencies which is long waves. -

Kuhn:

Now this was on the experimental or the theoretical... ?

Debye:

Well, the theoretical explanation was (???).

Uhlenbeck:

What did Mr. Ortvay do? Did he repeat these experiments?

Debye:

No, no, no. He came and I said, “Now this is what we have to find, out theoretically.” And so we talked about it from time to time.... My teaching activities were different, from those at Munich because I had a lot to do. I had to give the courses I was working in the lab with the people.

Kuhn:

You had done really little lab work with Sommerfeld?

Debye:

What there was of a lab was under my direction. I personally had not done very much. I tried to do the Stark effect, and such things, but in general not much came of it. In Zurich I tried to find dielectric saturation. I did not succeed.

Kuhn:

What was the reaction at the time to this permanent electrical structure in molecules?

Debye:

The dipoles? I don’t know. I think that was accepted fairly easily, especially because of the dispersion business. Well, they did not know what was the difference. Some things showed it, some things didn’t. And when it was shown that these things switch to the other pole, then it was all right.... You see the chemists only got interested in it really in my Leipzig time. It was after ‘27. Then they came in hoards from the United States. That was when Smyth and all these people were working there. (Smyth and Kurthold, and Fours, and Paul Gross) all these people, they all came over, and all wanted to do dielectrics. And oh yes, one of my friends in Wisconsin, 100 Williams. And then I was not interested in dielectrics anymore.

Uhlenbeck:

Did you know at that time Langevin?

Debye:

Oh yes, yes. I met him in Paris, but most of the time I met him at Solvay Congresses.

Uhlenbeck:

But it was after ‘13 then?

Debye:

Oh yes, after ‘13. Oh not at so early a time, oh no. Then I did only know off-hand through Weiss, because then I studied a little bit of magnetism. Then I came to Curie and Langevin and so on — all in Zurich....

Uhlenbeck:

Was it just one year in Zurich?

Debye:

The first time, yes. A year at Zurich, a year at Utrecht, and then on to Göttingen.... I told you why I went to Göttingen, because I could have the laboratory there. I did only teaching in selected topics, because Voigt wanted to do the regular things, and. I was very glad that he wanted to do the regular things. I had time for the laboratory and time to do some selected topics. I don’t remember now the topics, but I picked then always by asking the students “What would you want?”

Uhlenbeck:

And who else were there?

Debye:

At the time when I came then Reicke was still there as an experimentalist. And then Reicke died too• And then we had, to have another experimentalist. That took some time, so during a year I worked both things. We had, a lot of people coming over in order to look at them. We had also for instance Ehrenhaft there. He gave a big show, you know. Then we decided in the end that we should make a choice between Franck and between Pohl, both assistants of Rubens in Berlin. I told Hubert, Well, you better look at them. So I went with Hubert to Berlin, and we went to Rubens’ laboratory didn’t say why, you see — and met both of them. And then when we came back the general opinion was, “Well, it is hard to decide, but we want a man who really does a good. course in physics.” Pohl was doing all his experiments already, and so they decided Pohl was the man. So we got Pohl there as an experimental physicist instead of Reicke you see.

Uhlenbeck:

And Franck came later then.

Debye:

Franck was my successor. When I left, then they wanted a man who would do something with the lab and was a theoretical physicist. They asked Born. And then Born said, I cannot do anything with the lab, which is connected with theoretical physics. So why don't you take two people for this? So they took. Born and Franck, and Franck got the lab and Born was the theoretical man.... In Göttingen it was like this. You see Reicke was an experimental man; Voigt was a theoretical man. But each of them had half the laboratory. Each part independent of the other. And Voigt was doing mostly work on Zeeman effect. He did Zeeman effect and absorption.... And he was the specialist for crystal physics. He wrote a book about crystal] physics, but he didn’t do experimental work at my time....

Kuhn:

Now Laue was also there later.

Debye:

No, no, Lane was never in Göttingen. Laue was in Berlin, and he was helping Planck, and he had money and was rather independent. And I told you how he came to Munich. But Lane was later in Zurich for a short time. A place which I had and then he had and then Schrödinger had at the University. They had in Göttingen the Wolfskehl Foundation. The Wolfskehl Foundation was 100,000 marks as a prize for somebody who was going to prove some equation about numbers which Fermat had written down on the margin of a logarithmic table. And now the question was, how do we prove it? And so every year they got such papers which seemed to prove it. And then they had to be looked over by the mathematicians. And the one mathematician who was mostly doing it was Landau. Landau was a man for number theory. And they were all wrong. And Hubert decided, Well, this money is getting some interest, and so we should use this interest. Since Hilbert was interested in physics, he said, “Well, let’s make a congress.” Hilbert was going to spend the first 000 marks for a congress. And at this congress I talked about heat conductivity in crystals.... But that was the first one. And Hilbert said, Weil, people who come to these congresses should be paid like famous singers. And so he was going to get 1000 marks’

Uhlenbeck:

Lorentz was there too,

Debye:

No, no, he was not there. He may have sent this article in, you see, but he was not there. Kossel was not there.... Smoluchowski was there. Sommerfeld was not there.... One other man is missing — Grueneisen. Grueneisen was there and talked about the non-linearity of the motion of atoms in connection with expansion. ... You see the question is that an ideal crystal has no expansion coefficient.

Kuhn:

I wondered what the relation of that work was to the paper you had done, “Zustand-Gleichung und Quanten Hypothesis.’ You talk about the expansion coefficient f ideal crystals there.

Debye:

This was this congress, yes. That was the one with the heat conductivity. , This first idea that heat conductivity means is that you can have no heat 220 conductivity if you have not a free path length of the waves. And so I tried to calculate the free path. The one wave interferes with the other. And then you get no interference if you can linearly put them together, and so you have to have dependence of the amplitude which is more than the square of the displacement. Anyway, that was this congress.

Uhlenbeck:

That was in Göttingen in 1913?

Debye:

Now that was before I got there, you see. Then the next year, that’s when I came to it. The next year he asked me to go there. I think it was l914 they were there. Because then I was there, and then after a short time the war broke out. I was there before the war broke out. The war broke out, and I got my paper, giving my (installation) signed by Wilhelm. And Wilhelm was then at that time in France • I had to stay in Göttingen the whole time. Through the war. I got very thin because we had nothing to eat in the later years....

Uhlenbeck:

But it didn’t interfere very much with the scientific work.

Debye:

Yes, well those people who stayed on did some work. But you could not do experimental work anymore in the later years. For instance, the father of the wife of Joe Mayer, Maria Goeppert Mayer. . Her father was a medical man in Göttingen. He treated my boy — my boy was born just at that time. And then he needed some glycerin and he could not get any glycerin n anywhere. And so I went around the laboratory and looked around and sought and found a little bottle of glycerin for him, which he could use for his patients. That’s really how bad it was, you see.

Uhlenbeck:

We have just heard from Landé that all of these people were put somewhere in Berlin.

Debye:

That’s possible. You see I had nothing to do with that. Being a Dutchman I had nothing to do with that..,. There was development. But in the laboratories you could not do very much. Because there was just nothing. You could not get anything. Now I was happy because Scherrer was there, because Scherrer was Swiss. He had no trouble, so we could do our things. But even in the shop, when you wanted to get something made in the shop, you could get no material. You could get no brass or anything of that kind.

Kuhn:

To come back again to the Bohr atom. Do you remember the early days of the Bohr atom?

Debye:

Yes, I remember those because we talked about it, and we tried to calculate the dispersion of the hydrogen molecule, just calculating mechanically the vibrations which the ring would have....

Kuhn:

I was wondering whether you had any recollections of first impressions about the Bohr atom and the way it was received.

Debye:

Well, let me think back. I think everybody was saying — so far as I know — everybody was saying, well, this is a very nice idea and maybe it is right. And everybody was convinced really after you saw how the Balmer theories came out, that that was the way how you had to apply the quantum. At that time there was no question that if you wanted to do something more modern at that time you had to apply quantum. Quantum theory which did not exist, but you had to adjust so that you could use it. So that was prepared in everyone’s mind. So the application of the quantum was not the main point. The main point was really the combination of quantum theory as it was at that time with the picture.

Uhlenbeck:

You hadn’t met Bohr yet at that time?

Debye:

No, no.... He was just working at Rutherford’s laboratory.

Uhlenbeck:

You had not heard of him?

Debye:

No, no. Nobody had heard, as far as I am concerned. It just came out....

Kuhn:

You don’t remember the first time you heard of it.

Debye:

No. Certainly the first time I heard of it, it did not make a special impression.... Well, I had the feeling — when I reconstruct my feeling at that time — I had the feeling it was a very nice thing to do, and the main point is the model, not the application.

Kuhn:

What about the Rutherford atom? Had you been aware of that before the Bohr quantization?...

Debye:

Oh yes, because we had talked a lot about models, especially in connection with Hilbert’ s. interest for the seminar on the structure of matter, which I had to give. There we had talked about the other model of the positive sphere.

Kuhn:

Now that’s the Thomson model. But the ‘seminar would already be after the Bohr atom came out.

Debye:

No, not much.

Kuhn:

Not much, no. But I wondered whether, when you had been in Zurich when the Rutherford atom was suggested, I wondered whether you had heard of it.

Debye:

Well, in Zurich we did not talk very much about models. We talked about models in connection with the, seminar ‘in Göttingen. And tine the models were compared with each other. The model of Rutherford was compared to that of Thomson.

Kuhn:

This would already be in the Bohr version, though.

Debye:

Well yes, yes. But the first discussions were not of the Bohr versions. The first discussions I remember were of the alpha scattering. The alpha particles must be scattered in one (zoop), not a combination of a lot of things.

Kuhn:

You say you did not think much about models at Zurich, but were there physical models to go with permanent dipole moments?

Debye:

Oh, well you for instance take HC1, and if you have it in a liquid then it is an H ion and it s a Cl ion, but when they come together in a gas, they will still be positive on one side and negative on the other side, something like that, you see, but no attempt to get down to the atomic level. -

Uhlenbeck:

Why was Hubert interested in these things?

Debye:

You did not hear Hilbert was interested from this point of view. The first thing when I came there, he said, “NOW I am going to study physics, because I have the impression that now you have your electron, and the electron should be something which comes out of the mathematical equation as a result.” It's the same thing you are trying to do now. And so I am going to study physics if I can in the right way, so I can see whether I can make such an equation. That was his aim. So he decided I was going to do a seminar “Uber Struktur der Materie,” which still exists as a seminar in Göttingen. He always came there.,.. It met once a week.

Kuhn:

Would you lecture regularly?

Debye:

No, no, no We tried to have other people brought there, but there were always very good discussions.

Kuhn:

I’ve never been clear, in the German University, what the relation of a seminar to the colloquium was.

Debye:

Well, approximately the same thing, approximately the same thing. The main point was that there was talk about a subject and then there was discussion about it. Not that somebody stands there and preaches and then goes away. In the seminar the audience would be mostly students, but there was also faculty there. What we did in Munich was really a thing which Ewald introduced. We wanted a seminar, but a seminar in which there were no professors, because we wanted to be free to be as stupid as we wanted to be. And so we made that - - it was the idea of Ewald - - and so we made a seminar there, Ewald and I and Hopf and so on, you see. Then Sommerfeld said he wanted to come too, and then we said no, you cannot come. You can do something for us, you can give us cigars. And so he gave us a box of cigars

Kuhn:

But you told him he could not come?

Debye:

Yes, yes. -

Kuhn:

Could you say something about the paper on dispersion from the hydrogen molecule

Debye:

Well, it came naturally out of this seminar on the Struktur der Materie. At that time we had the Bohr atom. Then you say now if this is the Bohr atom, then you must also construct molecules. How would the hydrogen molecule look? And then you have the thing going around in a circle. Then you say well, let us take this as true, and then calculate some properties. So there you are.

Kuhn:

How did you feel about it?

Debye:

Well, as all those cases, you felt not quite sure, you know? But on the other hand it is not all bad.

Kuhn:

You say let’s calculate the properties, but you computed the value of 2 pi.

Debye:

No, no, no. You have this model. It is all decided. If you say that two electrons are going around like this, and then you have a mechanical problem. You have the mechanical problem that if you put it in an electro-magnetic field, how is it going to vibrate? Well that was simple....

Kuhn:

The quantum conditions do not appear in this paper until the very end.

Debye:

Well, maybe I put it in at the very end, but that is the meaning of the thing. You have a system in which the electrons go around with a certain velocity and so they are in a state there, in a dynamic state. And around this dynamic state you can have vibrations.

Kuhn:

It’s a lovely paper. It’s also one of the only, of course, that I think makes the dispersion problem as acute as it was.

Debye:

It is no good. You had the feeling that it was not quite right, but it was the best you could do. I did not know how to do it otherwise... But on the other hand, the practical results were not bad — for the index of refraction which you got out of that....

Uhlenbeck:

How about the Correspondence Principle?

Debye:

The connection with that really came from the Bohr side, absolutely from the Bohr side. He talked about it. He said, of course, “Well after all the old theory cannot be absolutely wrong, so there must be something if in the limit when h=o –“

Kuhn:

Who else in these years before 1920 at Göttingen was much concerned with the quantum contemporary development?

Debye:

Well really nobody. Really nobody. Wiechert was there, but he was much too interested in his earthquake business. Runge was interested but he was only interested from the point of view of his Runge-Paschen business. That was not the fundamental part. Well, there were discussions. Do you know about the Domir Spaziergang? In Göttingen there was a custom which had been introduced by Felix Klein. Every Thursday afternoon people came together and that was according to a definite scheme. Here is the (Wilhelm-Weber) Strasse. And there is a part of Göttingen, and that goes up to the mountain and then there is a restaurant there which is called the (Rontz). At one end of Weber Strasse Felix Klein was living. A little bit farther Hilbert, and next to Hilbert, Runge was living. So I think it was at just exactly quarter to 2 that Klein came out of his house, and went up slowly and the Hilbert had to fix the time so that when Klein was approaching his house he came out. He came always out with Pussy – Pussy was the dog. They went up and then Runge came out and so they went up there and then they went this way. Now we, and by ‘we’ I mean I myself and Carotheodory and then Prandtl. We met at a corner there at a definite time, and had to put our times so that we met together, synchronize. (Hersberger Chaussee) was the name there. There was Landau living, you see. And he had a wash room, which had a window you know, so he was always standing back of the window, and so when he saw us coming, then he rushed out of doors at the right time. Then the whole group was together, then we went up to the (Rontz), and at the (Rontz) all the faculty business was decided, independent of all the other peoples in the faculty ...And there were no chemists there.

Kuhn:

But the mathematicians and the physicists at Göttingen were extraordinarily close.

Debye:

Yes, yes. We were absolutely close. We decided what had to go on. We talked all about these other things, the Bohr atom and so on and so on. So that was every Thursday afternoon that we had coffee there and then we came back. So about 4:00 we came home. It was a leisurely afternoon. And that was the do-called (“Dominsen) Spaziergang.” And people like Abraham, and this other man, the astronomer ...they could not stand the (???Sohn) in Göttingen.

Uhlenbeck:

I had not realized that Abraham was there too.

Debye:

Yes, yes. There was a time when Abraham was there. Abraham was there earlier. Abraham and this other man, they were there as Privatdozent .. In Göttingen there was the custom that every student have his little booklet. And you had to testify in it. You had to put your name in at the beginning of the course and then you had to put your name in at the end of the course. And this second which you had to put in had the value that this thing was the last thing, that it could not be put in before a certain date. So these two boys, Abraham and the other one, I will get the name in a moment. They did not want to stay in Göttingen. They wanted to be in Munich. At that time I was in Munich. They wanted to come to Munich and have a good time in Munich. So they wanted to leave Göttingen as soon as they could. Now in Göttingen there is a restaurant which is called the Schwarze Bar, the Black Bear, you know. And so they went to this restaurant the last day when they could get their signature. Then the waited there until 12 o’clock midnight, their booklets were signed, and then there was the train. They got away from Göttingen. Then they appeard in Munich and then they were full of life.

Uhlenbeck:

Abraham must be a peculiar fellow, he was.

Debye:

Yes, he was quite a peculiar fellow, he was a little dry. He made good remarks but was dry.. He had trouble with Klein and everybody. He talked too much’. He said what he thought – everything. So he became then a professor in the northern part of Italy, in Milan.... He was the man who gave the second edition of this book about electrodynamics, Föppl And Prandtl was the son-in-law of Föppl.

Uhlenbeck:

I didn’t know that. But he was a very good man — Abraham — he must have been of genius caliber.

Debye:

Oh yes, he was a very good man, but he was nearly as bad as Zermelo.... He was very sharp. I would say there was more fun in what Pauli said than what Abraham said. Abraham was rather … just nasty.

Kuhn:

And Zermelo the same?

Debye:

Oh Zermelo. Yes, yes Zermelo was always excited and took everything the wrong way and so on.

Uhlenbeck:

Zermelo was in Zurich, wasn’t he?

Debye:

He was in Zurich. At that time I was in Zurich. He was not available because then he was up for his lungs somewhere. And then there was this man Haar.

Kuhn:

In this same period, there were the three papers — I guess all in 1916 — the dispersion paper, and the normal Zeeman effect paper. And then also fine structures....

Debye:

They would belong together as good as possible, but I don't remember. I would have to look that up.

Kuhn:

Two of them run very closely parallel to the work Sommerfeld is doing at exactly the same time.,.. These are quite independent in that you were not in contact with Sommerfeld?

Debye:

Oh yes. I had not contact at all, no. He felt that it was a kind of cutting into his domain.

Kuhn:

Did he say anything about this?

Debye:

Oh, no, he would not say that. But as soon as I talked to him, it came out.... As a matter of fact, in papers there was nothing special new in it. It was just the same method and same thing. You can say that you see.... But for him there was nothing new in it. For him there was nothing new, but the papers come out at just about the same time, on much the same problems.

Debye:

But it was his way of thinking, you see. As I told you, I did not want to do it anymore because he was doing it.

Kuhn:

The point you made this morning was that you think this happened because you were Sommerfeld’s student. Was it true of other people also, that the Sommerfeld approach was that marked?

Debye:

I don’t know, that depends of course on the people. If the people were a little bit mathematical minded, then they would do it that way. For instance Epstein, is just exactly —

Kuhn:

But at that time Munich was already in a sense the center for atomic theory, right?

Debye:

Yes. And Sommerfeld had introduced that as a special course, on top of the other courses ... and that was not done in Göttingen.

Kuhn:

But it’s true from these papers on, maybe from the year before on, but at least in the publication record these are the papers that make Munich the center for atomic studies.

Debye:

Yes. But it started because Sommerfeld gave this course. And for the course he had to study, you see. In doing that he published the papers....

Kuhn:

Did you see anything of Sommerfeld between the years ‘13 and ‘16?

Debye:

Oh yes, I would see him occasionally in summertime’s, because then we went down to Munich. There was no direct contact, writing of scientific matters or so.

Uhlenbeck:

You didn’t correspond with each other?

Debye:

No, no.

Kuhn:

So you knew very little of how his own interests were developing, the Bohr atom...

Debye:

Oh yes, very little, very little. It was a little bit of a surprise to me you see, how he took it when I saw him. That was all right....

Uhlenbeck:

Why did you go back to Zurich?

Debye:

You see, it was absolutely impossible in Göttingen then, at the end of the war• The French had decided that they wanted - - well, you know how the French gravitate to Paris. And everybody has to be in Paris, and if he is not in Paris he doesn’t count for anything. And so they decided that was no good, and so they were going to make Strasbourg a very good university which would be a counterweight to Paris. And there they had asked Paul Weiss, Pierre Weiss, to go. Because he was a Frenchman anyway. And then under the influence of Stodola, you may have heard of Stodola. He was a very good man at the Technische Hochschule in Zurich. He was a Hungarian, and was more of an engineer, but very much interested in relativity, very much interested in all mathematical things. And he wrote a book for instance about turbines. He wrote that right from the thermodynamics point of view and so on. He decided that they had to have me. And so then they asked me to go there, and then of course Scherrer went with me.... And that was really a relief, because it was impossible to do anything experimental at Göttingen.... So I got there as the experimental physicist so to say. And I brought Scherrer with me• And then I was thinking about this electron business. And so I tried to convince Scherrer he should do something about it. He should see if the light had really changed its frequency, by applying not the real picture, by applying only the law of conservation of energy, the law of conservation of momentum.

Uhlenbeck:

Can you trace a little bit at what time this idea became familiar to you?

Debye:

Well, that must have been the first years, because I had published something about that • And when I published it, it must have been two years at least after I got the idea, because I waited a. long time that Scherrer should do something about it.

Kuhn:

When you say you published, you mean you published immediately after the Compton paper came out with the experimental results?

Debye:

Yes, yes. No, I published it, and then I found out that Compton had already published something. So during the time I was publishing it I found out that Compton had done it.

Kuhn:

Well did you have two papers?

Debye:

No, no • In that paper you will find, the remark —

Kuhn:

Well you start that paper by saying this has been on your mind for some time, and you refer to Compton for the data and so on.

Debye:

But the fact is you see that I was thinking of this. I told Scherrer, We have to do something about it, we just did not get to it....

Uhlenbeck:

But that of course implies that one has this idea of the momentum of the photon

Debye:

Oh yes, but that was the first thing, you see.

Uhlenbeck:

And now that is of course implicit in Einstein’s fluctuation papers.

Debye:

It is also implicit in the pressure of the light.... not as a photon. But that is clear, you see.... As long as you did not specify the energy as h, there was just E and E divided by c is the momentum....

Uhlenbeck:

But it’s an extremely small effect, it took so to say a certain step in thinking to take it seriously that the photon had also momentum. And you did it so to say purely from theoretical —

Debye:

Purely from the theoretical point of view, and then I told Scherrer, there must be this thing and we should do an experiment in order to show it. Then later on we found out that the experiments had been done.

Kuhn:

Well, you said this morning when the thing came up, that there was the old problem of the breakdown of the .2 factor in the scattering formula.

Debye:

Yes, yes. That is what we knew, because we had been involved in light scattering and X-ray scattering the whole time, and so we knew about Wagner and this fact that the scattering coefficient divided by the density is always 0.2, except if you go to high frequencies. And in order to find, some reason for that, for this breakdown which you could not get classical at all, I thought, “Well let me try this business with the h’s and the momentum.” That was the real beginning....

Kuhn:

This morning you rather indicated that in trying this out you weren’t really thinking of photons.

Debye:

No, no, I was not thinking of — I said, “I don’t want a mechanism, I just want general laws and see what the general laws say,” because at the end you can always put a mechanism in • You know there was the time when some people were really thinking that energy conservation would be common to average but not for every process....I’m not saying that this was a thing which induced me, but there are people very soon afterwards then who are thinking of that.

Kuhn:

I was wondering whether people were already thinking of possible breakdowns of energy conservation.

Debye:

No, no, no. That did not play any role.

Kuhn:

That really comes out of another problem. After these‘16 papers, except for the Compton-Debye effect.

Debye:

Oh, don’t call it that. There is no reason for that. There are a lot of things which happened at the same time. Like the magnetically low temperatures. So you should not do that. The man who does most of the work, he should get the name. No, some people in Germany have done that. You know how the Germans are — like the Russians.

Kuhn:

You don’t after this have very much to do yourself with quantum effects.

Debye:

Well, I had to do with this low temperature business. That was at approximately the same time • That came out of the question that if you take Langevin, then you cannot get entropy zero, because then you get a logarithmic infinity. So there must be a difference. And then I said how far can I do it? I took the experiment of Kamerlingh-Onnes, and then I said, “Well, I can go a lot farther.” That means that I can really get practical temperature differences.

Uhlenbeck:

I mean this was before the space quantization?

Debye:

Yes, yes. I think so. It has nothing to do with space quantization.... What I said was only that if I take the entropy, which I can get from the magnetization curve, then if -this magnetization curve is the Langevin curve, then I get a contradiction to zero entropy for zero temperature. So that cannot be true. Now let us see how far I can go. And at that time Kamerlingh Onnes had already done measurements about the paramagnetic saturation at low temperatures. Then I put that in. And so I know in what range I can use my entropy.

Uhlenbeck:

Simply empirically.

Debye:

Empirically. Then I also know what the specific heat is, which is very low. So if I combine these things then I come to the result that if I do the experiment I certainly can get lower temperatures than 0.2 degrees. That was allNo, no, no quantization! But I knew there had to be quantization.... I tried something about that. But I did not put any quantitative thing in there. Just this thing, you see. Knowing that this quantization has to get in at lower temperatures still, you can do something practical about it.

Uhlenbeck:

May I at one point go back? When you were in Göttingen of course, the Einstein paper of 1917 appeared on the continuous emission and absorption. Was that a kind of revelation?

Debye:

Not in my memory.... I know that we talked about it, but it made not the big impression as something very strong does, at least in my memory.

Uhlenbeck:

And while you were in Göttingen also, were you or were the people very much interested in general relativity? ... Hilbert must have been.

Debye:

Hilbert. For Hilbert everything was finished with Minkowski.

Uhlenbeck:

But not the general relativity.

Debye:

Not general relativity, no... But you see for those mathematicians that was something which was not - - you know, it was something which was (“gang gàbe”). You see Einstein had to learn that business, and he learned it from this other mathematician in Zurich at the Technische Hochschule, Grossmann. He learned it from him. So for the mathematicians it was nothing especially new.

Uhlenbeck:

And physicists were not interested really?

Debye:

Well there were no physicists. Well, I was there. But then there was one by the name Rausch von Traubenberg. He was an experimental physicist. People like Pohl were not interested in it. It was just experimental. Hertha Sponer was there at that time. She is now the wife of James Franck, you know. Hertha Sponer got her Ph.D. degree with me in infra-red, in Göttingen. And there was another girl there, who was an Austrian. She died later. Some of these then were at the colloquium and talked about these things, but they were also students.

Uhlenbeck:

You were not in personal contact with Einstein?

Debye:

Not very, very little. I don’t know whether he was in Berlin at that time. But during the time in Berlin I never saw him. No, there was a little bit of bad feeling there. I was a good friend with (Sank), of whom I told you, at Zurich. (Szanka) had some trouble with Einstein because Einstein had divorced his wife. And they had three or four children, and Szanka did not like that. He left them, so to say, for this other relative in Berlin. I know too much about these things, and that was not right.

Kuhn:

Your earlier years in Zurich are just the period in which, in a few places, at least, people are beginning to express again that feeling of frustration which you spoke of this morning in connection with the early attempts to assimilate the quantum. Now it’s that the quantum theory is wrong. Was this the sort of thing that was discussed at Zurich also?

Debye:

No, in Zurich, no no. Not in Zurich.

Uhlenbeck:

Was there a theoretician at Zurich at that time?

Debye:

No, there was none.

Kuhn:

Not at the University yet either?

Debye:

The University had no theoretician before Einstein was there. And so I was the only one who was representative officially of theoretical physics, and there was nobody at the Technische Hochschule. No, there was no connection there. The only connection I had during that time was with Haar, who was a mathematician.

Kuhn:

Did you teach the same courses when you went to Zurich the second time?

Debye:

No, no. When I went the second time I went as an experimentalist, and I introduced these different courses for different types of engineers. I told you. For one of these courses I got the professorship for Scherrer. And at the first period I was there then I taught for electrical engineering for physicists, and Scherrer taught for chemists, and Picard — you know the one who always went up with the balloons - Picard was one man left over from Pierre Weiss, and be taught for architects and so on. And later on there was a man by the name of (Tunge) who did it. (Tunge) is still there.

Uhlenbeck:

And who were the mathematicians?

Debye:

(Polya) was there, but I don’t know whether that was at that time or not. I could look it up...Speiser was there. There was a man with a French name. He was at the Technische Hochschule, and I don’t remember now. Because there was really not much —- they never came to seminars. The seminar was a combined seminar with the physicists of the Technical Hochschule and of the physicists of the University.... The mathematicians did not come. Weyl may have come now and then. You see Schrödinger got those things really - - well, because there was this publication of De Broglie. Then we read it, and then I said to Schrödinger, “Schrödinger, you know, I have not much time.” It was just the same as it is now in this respect, because we had so many other things to do, and I don’t understand this De Broglie business. You read it. You see if you could get a nice talk about it.” And this — reading De Broglie brought Schrödinger to these things. Now there is already a background for him.

Kuhn:

How did people receive the De Broglie paper?

Debye:

Well, they had the feeling that — you see there was this question of the thing dragging in its own tail you know, and going around. And it is not the right thing.

Uhlenbeck:

That was ‘24 already.

Debye:

Yes. But the other paper — the fact that you get a wave length connected with something going, like, an electron, lambda is h/p. This was something which made a big impact.

Kuhn:

But how seriously was one supposed to take this in the absence of experimental evidence? ,

Debye:

Well, the experiments came very soon after. I heard about it when I was traveling in the United States. At that time I was giving a course at Madison, Wisconsin. On the train going from Madison, Wisconsin, to Washington, an experimental physicist was there. He told me about it— that he had heard about it. So we tried to reconstruct it. What kind of wave length should be connected with momentum? And we came to the right formula’. So you see as soon as you have the idea that there should be a connection, then it was easy to construct it.

Kuhn:

Do you remember when that train ride would have been?

Debye:

I was going to a meeting of the American Physical Society in Washington, but that is all. That was during my time I was at Zurich, I know that

Kuhn:

Do you remember the Elsasser piece in Naturwissenschaften just a little note?

Debye:

No....

Kuhn:

Had you known of the Ramsauer effect?

Debye:

Yes, we heard about that. Yes, yes.

Kuhn:

Because apparently some people were deeply bothered by that one, other people not so much.

Debye:

I must say that in my memory there was not a good explanation of it.

Kuhn:

There was a real interval in which in many places people had not even heard of the De Broglie paper. And in other places people had heard of it but were disposed to dismiss it.

Debye:

In my memory the first impact it had on me was this trying to find out what kind of a relation it could be. And we worked it out on the train, I know that. Then after that very shortly - - in my memory very shortly — I heard about electrons going through gold films.... What I heard on the train was an experimental physicist from Madison, Wisconsin, sitting with me there going to the American Physical Society meeting in Washington, telling me that he had read or heard that there was a connection between the wave length of a bunch of electrons and their velocity.

Kuhn:

An experimental - -

Debye:

No, no. He had heard about the theoretical connection.

Uhlenbeck:

That must have been — De Broglie did that.

Debye:

Yes, yes. And we tried to reconstruct what kind of a connection that could be.

Uhlenbeck:

But you had so to say the De Broglie thesis in your hand at a certain moment? You gave it to Schrödinger.

Debye:

Yes, yes.... Schrödinger studied it, and then.... He gave a talk on it. And then very shortly afterwards the experimental physicist at the University —-his name was Meyer - - came to me and said, “Now Schrödinger has something very fine.’ And I tried to distill out of him what it was that was so fine....

Uhlenbeck:

And now, about these years, there was of course this whole period and debate about atomic structure, and about the vector model and about spectra and so on. In which sense was it localized? Was it only in Munich and Göttingen that people worried about it?

Debye:

Yes, yes. From my feeling it was too complicated.

Uhlenbeck:

And it was also in Zurich not a topic of discussion at the time?

Debye:

No, no, no. No, in Zurich they were much more interested in general connections, not in the details of the spectrum. You know there was a time when that was the only thing physicists were interested in, the spectra.

Uhlenbeck:

In some places. And that was only Munich and Göttingen?

Debye:

Yes, yes.

Kuhn:

What about the other great developments of this period, the Stern-Gerlach experiment?... There’s a bad paradox in this one.

Debye:

Yes, but I cannot remember that this had an impact in my surroundings.

Uhlenbeck:

Were you still in Zurich when the Schrödinger equation came out?

Debye:

Yes, oh yes.

Uhlenbeck:

Oh, you were still there? Tell us something about that.

Debye:

I told you just a moment ago. I heard about it from Meyer.

Uhlenbeck:

Was it presented at a colloquium? -

Debye:

Oh yes, oh yes. It was presented in colloquium, and the next thing again, and so on. It was considered as really a big thing. I must say, I myself — I was a little bit relieved. Because I did not like the Heisenberg approach.

Kuhn:

Go back and tell us first about that.

Debye:

How do you mean, “tell us about that?” I had nothing to do with the whole business.

Kuhn:

No. But the initial Heisenberg paper — did people pay attention to this before the Drei-Manner-Arbeit?

Debye:

Yes, yes. They paid attention to it. And then you had the feeling, why this complicated mathematics and so on. And as soon as it was put in a differential equation, and connected with the mechanics - - the ds/dt=H, you know — then of course, at least I felt a lot relieved. I felt that then I understood things. Whereas with Heisenberg I did not

Uhlenbeck:

But you didn’t talk with Schrödinger about it, in statu nascendi, when he first had these ideas?

Debye:

He did not talk about it at all. He did not talk to anybody at all. Except he went to the mathematician, Weyl, in order to help him out with some of the equations.

Uhlenbeck:

He worked by himself?

Debye:

Yes....

Uhlenbeck:

And although one knew him, one had never heard very much of him before.

Debye:

Well, yes. Otherwise he would not have been there in Zurich, if he had not a good name.

Uhlenbeck:

Yes, but in a much more specialized sense.

Debye:

I always, had the feeling that he had a type of thinking already in his Antritts-Vorlesung, when he was talking about the (natural) fluctuations, independent of the statistics. And of course he went out more to this connection with mechanics, of the Hamilton business....

Uhlenbeck:

(Exner), wasn’t it (Exner) in Austria who bad these philosophical ideas?

Debye:

Well, he brought it over from Vienna.

Kuhn:

Were you there for the Antritts-Vorlesung?

Debye:

Yes, yes.

Kuhn:

How did people react? How did you react to this idea of natural fluctuations?

Debye:

Well, we said, “Who knows.” We are in such a trouble with quantum theory, when we get in, the most frightful things may come out of it. You see, all these things which you did in quantum were guessing. You had no general thing to get it from.

Uhlenbeck:

But your interest at that time, between 2O and ‘24, what were they mainly? Was that already the time of the electrolytes?

Debye:

No. That was the time of the electrolytes, but that came ‘about very queerly, because I was not interested in electrolytes at all. We had some colloquia together with the physical chemists, and they were not held at the University, but they were held at one of these Zunfthäuser, these houses of the guilds. And there was one at the (Limmat), and we came together, oh, every two or three months perhaps. Then somebody was talking. There was a man, what was his name now? His name was Bauer and he was the physical chemist at the Technical Hochschule. And he is quite well-known, because he was at that time a specialist in fuel cells, which are now coming up again. So he had been in Berlin and had met Nernst. And Nernst had told him about a new development in physical chemistry .In physical chemistry they had always had this trouble with strong electrolytes, because (Arrhenius-Ostwald) did not apply at all. And in old times, long, long before the end of the century, Kohlrausch had measured conductivities and had found that the conductivity goes with the square (root) of the concentration, and had written. This is a thing which is absolutely un-understandable from the (Arrhenius-Ostwald) theory.. On the other hand, it is such a nice law, that it must be the right one, this is what he writes, you see. So I had a long think about that, of course. This man Bauer, he comes now and gives a talk. And he starts his talk by saying, “Now we have long had old shoes” — I still remember that — “old shoes in physical chemistry, to walk on. But now we have a pair of good new ones. And there is an Indian who has delivered them, an Indian by the name of Ghosh “His picture was that you take a sodium chloride solution, then the sodium and the chloride in the solution are in the same position with respect to each other as in a crystal. With increasing dilution, the distances get bigger and bigger. And from that he derived a third root law, you see third root of the volume, you know? So that was what the new shoes were. And Nernst believed in that. And so in this place I said, that cannot be possible, because these things must have Brownian motion. And I got in trouble with them, and we were a little bit heated up over that. (They) said, “Well, if you really mean that, you will have to do something about it.” So I went home and did something. That was all, you see. But then I did not know about the literature. So I said to Hückel who was my assistant, “Now you, boy, you study the literature and tell me about it so then we’ll see what we are missing.” That’s how the whole thing came about.

Kuhn:

This suggests a question I have meant to ask you earlier. Which was how the whole earlier development of the X-ray work started?

Debye:

Oh, the X-ray work. That started really before Utrecht. After Laue had done his things, I said, “Well, now you know it is not true that the atoms are standing still. They are moving. So what is the influence of these atoms. And in connection with that, why is it necessary to. have a crystal structure? Why cannot any geometrical structure be good enough? And from that I published a little article about atoms connected with each other and scattering by their electrons. And about the same time, at the same day, Ehrenfest presented the same kind of thing for a diatonic molecule, and it just turns out that the time I presented it at Göttingen and the time his was presented in Amsterdam are just exactly the same day’. So it all started with that, you see.

Uhlenbeck:

With, so to say, the X-ray scattering of gases —

Debye:

Yes, of anything, anything.... And so when I came to Göttingen, I said, I am going to do experiments. And I made this little camera and put everything in. Everything under, the sun I could think of. I still remember on these (Domisen) Spaziergängèn I showed some films and so on. And then one thing. Then we had an idea. We thought we could get something about the structure as far as electrons were concerned. We said, “Well, maybe it is the best thing to do something experimental, to take things with as small a number of electrons as possible.” And so in order to have a thing with a small number of electrons, we took lithium fluoride. And put that in.

Uhlenbeck:

The crystal?

Debye:

Powder, powder, we did not know there was a crystal’. Powder Like other powders which we had put in. And so then Scherrer exposed it, and then developed it and comes up to me and says, “See, look here. We got a lot of lights.’ Well, it was very clear very soon, you see, in a few minutes, what had happened. But we had not done that. We had not thought about crystals. That was in Göttingen. And then we published that in the academy there, because we could get something published in fourteen days .,.. The Göttingen Academy. The Academy put everything in. Then I thought I could see now the structure of atoms, molecules, and so I took liquids. With the help of liquids we got also maxima and minima. We made the wrong interpretation of benzene. We said, Well, what we see there may be the structure of the benzene molecule.” Then later on it was clear that that was just the distance of benzene to benzene. Then of course when that was clear, then we had to do something with gases. That I could do only after I was in Leipzig.

Uhlenbeck:

But while you were in your second period in Zurich, was this your main attention? The X-ray scattering?

Debye:

No, I cannot say that. I mean we talked about this electron business; we talked about magnetic cooling.... We had two things in mind. We had in mind the magnetic cooling, and we knew that we had to go through the scattering in gases. And then I went to Leipzig.. And in Leipzig, with the government there, the Saxon government, I said, “Well, all right You get me money for doing these two things, first of all magnetic low temperatures; secondly let me do the experiment with gases. And they promised to get me the money for those things. Then it came to the effect that I got all of the money for one thing.. And so I decided I am going to do the gases. But these were the two things which I wanted to do at the new place.

Uhlenbeck:

And. you couldn’t do that in Zurich, so to say?

Debye:

Well, had I stayed in Zurich I still could, do it, but then they offered such big possibilities at Leipzig, which did not really come out at the end. But I said, well, all right, I’ll have to do that. And also, I had the feeling, “Well Scherrer is now very good, and he can do all the things,” so I wasn’t needed at Zurich anymore. So I did not do anything about the magnetic business and that went over to de Haas.

Kuhn:

Did you ever get involved in the magnetization work with the question of the size of the magneton, or the existence of the magneton?

Debye:

No, no, I did not get involved in that. At the Solvay Congress there was this man from Spain, Cabrera who was talking about the magneton, and then of course Weiss was talking about magnetons and so on. But we never wanted to believe that, really. They had these curves which were never very cunning.

Uhlenbeck:

Do you remember the first Solvay Congress that you attended?

Debye:

Well, I don’t know that it was the first, whether I remember just the first. I went to several of them. .

Uhlenbeck:

Because they have been in certain periods a great influence.

Debye:

Well, yes. The trouble with the Solvay Congresses was always that the publication came too late.... The meeting itself was very g00d, very good. There you could talk to each other.

Uhlenbeck:

Because there were these great debates of Bohr and Einstein, and later Heisenberg and Pauli.

Debye:

Yes, yes. Well, that was the point. Also, do you remember that they always wanted to keep these things very secret?

Uhlenbeck:

No, I do not remember.

Debye:

Oh, you don’t? Well you see the point was that they were bringing people together, and that those people should speak freely. They could only speak freely if there were no other people around. And that was a thing which irked professors at the Belgium universities. And so they said they wanted to be also present to hear something which they could use. And in the end it was decided they could come, but they had to sit in another part of the room and could not take part in the discussion So you see it was really secret, in order to give the people who were there — Rutherford and Bohr and all these Englishmen too — to give them an opportunity to say anything as stupid as they wanted, without being embarrassed.

Kuhn:

Did that work, that technique?

Debye:

Yes, it worked. It worked all right.

Uhlenbeck:

Then they were very small too.

Debye:

Yes, perhaps 20, 25. But it did not work for the general public, because it was published much too late. It was years and years old. Now I tried to do something different you see with this Houston business. Which has been conceived really because of the Solvay Congress. But what I do there, I get people over for different subjects. Six people really. You see each has half a day. Bat there is free discussion, and there are about 20 to 25 people of the United States invited. Anyone of these men, who come from anywhere in the world, gets $1000 for the talk and everything free. So I have good people, and I have an audience. So the rest is open, open to the public and nobody has to pay anything. They can even get a publication without paying anything, they just have to write for it. Now it’s getting too big now. The last time we had 700 people!

Uhlenbeck:

But the discussion is only between the group of people?

Debye:

Well — everybody can say something. They warned me, “We’ll have trouble with that. But it has been five or six times now, and never any trouble, because nobody dares say anything, except to people who are really — you know them.

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